The basic feedstocks for the production of first generation biofuels are often seeds, like grains such as wheat and corn, that produce starch or sugar cane and sugar beets that produce sugars that is fermented into bioethanol. However, the production of ethanol from these feedstocks suffers from the limitation that much of the farmland which is suitable for their production is already in use for food production.
Biologically produced alcohols, most commonly ethanol, and less commonly propanol and butanol, can be produced by the action of enzymes and microorganisms through the hydrolysis of starches or celluloses to glucose and subsequently fermentation of sugars. Cellulosic ethanol production uses non-food crops and does not divert food away from the food chain or inedible waste products which does not change the area of farmland in use for food products. However, production of ethanol from cellulose poses a difficult technical problem. Some of the factors for this difficulty are the physical density of lignocelluloses (like wood) that can make penetration of the biomass structure of lignocelluloses with chemicals difficult and the chemical complexity of lignocelluloses that lead to difficulty in breaking down the long chain polymeric structure of cellulose into sugars that can be fermented. Thus, it requires a great amount of processing to make the sugar monomers available to the microorganisms that are typically used to produce ethanol by fermentation.
Lignocellulose is the most abundant plant material resource and is composed mainly of cellulose, hemicelluloses and lignin. Woodchips are used in pulp and paper mills to convert wood into wood pulp by chemical or physical processes, usually Kraft process. In a Kraft process, woodchips are treated in a digester with a mixture of sodium hydroxide and sodium sulfide, known as white liquor. The woodchips are impregnated with a cooking solution that contains white liquor. White liquor is produced in the chemical recovery process.
In a continuous digester, the materials are fed at a rate which allows the pulping reaction to be complete by the time the materials exit the reactor. Typically delignification requires several hours at 155 to 175° C., typically around 170° C. Under these conditions lignin and some hemicelluloses degrade to give fragments that are soluble in the strongly basic white liquor. The solid pulp (about 50% by weight based on the dry wood chips) known as brown stock is collected and washed to produce brownstock pulp that typically contains 3 to 4% by weight lignin (Kappa #20-30) for softwood and 2 to 3% by weight lignin (Kappa #10-20) for hardwood, which is further passed through a series of bleaching steps to generate paper-quality pulp. The combined liquids known as black liquor contains extracted lignins, carbohydrates, sodium hydroxide, sodium sulfide and other inorganic salts. The black liquor is at about 15% solids and is concentrated in a multiple effect evaporator to 60% or even 75% solids and burned in the recovery boiler to recover the inorganic chemicals for reuse in the process. The combustion is carried out such that sodium sulfate, added as make-up is reduced to sodium sulfide by the organic carbon in the mixture. The molten salts from the recovery boiler are dissolved in process water known as “weak white liquor” composed of all liquors used to wash lime mud and green liquor precipitates. The resulting solution of sodium carbonate and sodium sulfide is known as “green liquor”. Green liquor contains at least 4 wt %, typically 5 wt %, of sodium carbonate concentration. Green liquor is mixed with calcium hydroxide to regenerate the white liquor used in the pulping process.
Currently there exist two broad categories of processes for the hydrolysis of cellulose. One category uses mineral acids such as sulfuric acid as discussed in U.S. Pat. No. 5,726,046, while the second category uses enzymes. The mineral acid most commonly used in mineral acid process is sulfuric acid. In general sulfuric acid hydrolysis can be categorized as either dilute acid hydrolysis or concentrated acid hydrolysis.
The dilute acid processes generally involve the use of 0.5% to 15% sulfuric acid to hydrolyze the cellulosic material. In addition, temperatures ranging from 90° C. to 600° C., and pressure up to 800 psi are necessary to affect the hydrolysis. At high temperatures, the sugars degrade to form furfural and other undesirable by-products. The resulting fermentable sugar yields are generally low, less than 50% and process equipment must be employed to physically remove furfural before further processing.
The concentrated acid processes have been somewhat more successful, producing higher yields of sugar. However, these processes typically involve the use of 60% to 90% sulfuric acid to affect hydrolysis, leading to high cost due to the cost of handling concentrated sulfuric acid and it subsequent recovery.
The additional problems faced in the acid hydrolysis processes include the production of large amounts of gypsum when the spent or used acid is neutralized. The low sugar concentrations resulting from the processes require the need for concentration before fermentation can proceed. When hydrolysis is carried out at temperatures above 150° C., compounds such as furfural are produced from the degradation of pentoses. These compounds inhibit fermentation, and some may be toxic. Furthermore, the degradation of pentose sugars results in a loss of yield.
U.S. Pat. No. 4,070,232 describes the prehydrolysis step in the presence of dilute acid solutions containing a mixture of HCl, formic and acetic acid which is pretty corrosive mixture requiring expensive process equipment. Also, the recovery of hemicelluloses is low due to short residence times (7-20 min) at low temperatures (100-130° C.).
US2008/0190013 describes use of ionic liquids to pretreat lgnocellulosic material. However, ionic liquids are generally more expensive and difficult to recover, while cleaning (building-up of heavy components) is required. Minor losses will make the process uneconomical.